Add gitter button (#20)

README.md

@@ -1,3 +1,5 @@
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/vaadin?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+
 Vaadin is a web development platform that enables you to create modern and fast web applications.
 
 Note: https://github.com/vaadin/vaadin was earlier the main repository for